A Railway post office, or RPO, was a traveling post office. RPO cars were attached to regularly-scheduled passenger trains, and were operated under contract to the US Postal Service. Mail was picked up and dropped off at cities and towns along the route, and sorted while the train traveled. By the mid-1940's, there were 1,500 RPO routes criss-crossing America with 30,000 men working in more than 4,000 RPO cars. An average crew numbered 12 men. More than 90% of the nations mail was handled in RPO cars.